Crustaceans are a group of animals with their skeletons on the outside. Other groups of animals that share this feature are insects and spiders. Crustaceans have gills or modified gills so they can live in water or damp places. Crustaceans can have many legs and up to two pairs of antennae. Most have eyes.
Once crustaceans hatch from their eggs they go through several larval forms. To grow, crustaceans are like insects and spiders and must shed or moult their old skeleton. The new skeleton has started growing underneath the old one and is still soft. It is inflated into a larger skeleton and hardens after moulting.